Post-stressed concrete structure



Feb. 19, 1957 Original Filed July :51, 1951 C. DOBELL POST-STRESSED CONCRETE STRUCTURE 3 Sheets-Sheet l INVENTOR f (T161200 Mel! v 20 ATTORNEYS 2,781,658 POST-STRESSED CONCRETE STRUCTURE Curzon Dobell, Bronxville, N. Y., assignor to Stressteel Corporation, New York, N. Y., a corporation of Dela- Ware Original application July 31, 1951, Serial No. 239,423.

Divided and this application August 28, 1952, Serial No. 306,862 7 2 Claims. (Cl. 72-50) This invention relates to concrete objects characterized particularly by their great strength and relative simplicity of construction.

It has long been common to reinforce the tension zones of concrete with steel but only in recent years have engineers learned to obtain any appreciable degree of chiciency from steel that is used for this purpose. Even yet the constructions in which steel is used with a fair degree of efiiciency are complicated, costly, and can be undertaken only by experts in the field.

The present invention is directed toward the production of concrete objects in a far simpler, easier, less costly manner and yet in a way that will produce concrete objects having great resistance to bending stresses even in the hands of relatively unskilled persons.

For many years, mild, low strength steel reinforcing was simply embedded in ordinary concrete and the result was an improvement in the strength of the concrete and some resistance to cracking and other failures. Later there was an attempt to place this reinforcement under tension so that it would have a compressive effect upon the concrete. This slightly improved the construction although the success of this effort was not great because those who did it did not appreciate the fact that concrete would flow slightly under pressure and thus that the tension on the reinforcement would be reduced practically back to zero. Also, under tension the reinforcement itself would suffer some permanent elongation that would add to this reduction of the tension.

Accordingly, the tensioning of the reinforcement was not generally considered to be of great importance until recently, when some of the factors influencing the elfectiveness of reinforcement became better known and it was found that if steel wire having very high tensile strength was used and was placed under a very high tension, then, although a part of this tension was lost by plastic flow and shrinkage in the concrete and permanent elongation or creep in the wire, nevertheless, sufiicient tension would remain in the wire and sufficient compression would remain on the concrete to cause a major increase in the load carrying capacity of the concrete and a major increase in the resistance of the concrete to cracking and the like. This represented such a complete de parture from previously known facts as to present a completely new concept of reinforced concrete and hence, this high tensioned wire reinforcing of concrete was properly considered to furnish a completely new and different approach to reinforced concrete construction.

In the reinforcing of concrete by these high tensioned wire arrangements, many methods and techniques have been developed because the handling of the great number of wires necessary to reinforce any fairly large concrete object-is extremely intricate and tedious and requires expert supervision if the wires are all to be tensioned and anchored properly so that there will be no warping or cracking or weakening due to unevenness of the reinforceme nt tension itself. This high tension wire rein- 2,781,658 Patented Feb. 19, 1957 forcing has now been practiced for several years without substantial change and in the belief that its complication and detail are necessary adjuncts to the production of high strength reinforced concrete objects.

According to this application, however, it has been discovered that much of the tedium and detail and difliculties of high tensile strength wire reinforcing can be avoided by the reinforcing of concrete, under proper con-- ditions, with high tensile strength rods as distinguished from wires. These rods can carry many times the load of individual wires and hence, far fewer rods need be used than wires. They are not subject to twisting and tangling and other disarrangement as are wires and can be handled by relatively less skilled labor and with relatively less skilled supervision to produce equally good, if not better, results, all at less cost and in an easier manner.

The transition from untensioned or slightly tensioned reinforcement to highly tensioned wire reinforcement and now to highly tensioned rod reinforcement in concrete comes not as a natural sequence of development but rather as sudden, drastic, and rather complete changes in the concept of strong concrete construction.

Thus, at the time that the possibility of using high tensioned wire reinforcement came into being, the general idea of tensioned reinforcement had long been known and was considered of relatively little value. The discovery that high tensioned wire reinforcement could produce far stronger and more durable structures than had ever before been known was a discovery that revolutionized the manufacture of reinforced concrete structures. Now, while people have known for some years that high tensioned wire reinforcement of concrete was practical, the

known or understood until the present invention.

There are several facts that may account for the failure of those in the art to propose and appreciate the advantage of using rods under high tension to reinforce concrete. For one thing, the elastic properties of high strength rods are not as generally appreciated as are the elastic qualities of high strength wires, and high strength rods themselves were not common until recently. Thus, although engineers understood the possibility of stretching a wire sufliciently so that its elasticity would take up w the shrinkage and plastic flow of the concrete and still leave enough tension in the wire to place the concrete under compression, an engineer would not have ordinarily thought of a rod as having suflicient elasticity for such a purpose. Then too, by the use of a series of wires the compressive load placed on the concrete could be distributed over a relatively large area of concrete, whereas with a rod, of considerably larger diameter, the load would be concentrated on a relatively small area of concrete and it, apparently, was not previously appreciated that the concrete could be made to sustain such compressive load. Still in addition, the apparent difficulty of holding the ends of a bar when it is under a high tension presents quite a problem that the prior art did not know how to solve. There is no indication that any attempt was previously made to use highly tensioned rods in concrete nor that the advantage of so doing was appreciated, but the above difiiculties indicate some of the problems that would have faced anyone, prior to this inventor, that attempted to do so.

The present invention relates to concrete prestressed and reinforced by rods under high tension and in order to produce such an object, the difliculties enumerated matter of size, wire practically never exceeds the diameter of one-fourth inch whereas rods are"practically never less than one-fourth inch in diameter.. Wire is formed by drawingthrough a die Whereas the rod is formed by rolling, or some process similartorolling, which is quite.

different from the drawing process involved in the making of wire; Wire is commonly sold in rolls or on spools whereas rods are invariably produced in straight lengths. Theirods to be used in accordance with the principles of this invention are always at least one-half inch in diameter and will not usually exceed one and a half inches in diameter.

The concrete objects in accordance with this invention may be of any. desired shape but are generally beams, columns, channels or other structural members. In order to obtain substantial advantage from the principles of this invention, it has been found necessary to make the concrete itselfstrong and dense and it should have a standard compressive strength at 28 days of at least four thousand pounds per square inch and prefer-ably eight thousand pounds per square inch or over. The reinforcing rods have anultirnate tensile strength of at least two hundred thousand pounds per square inch and preferably over two hundred and fifty thousand pounds per square inch.

After the concretehas gained its strength, the bars should be given an initial stress of at least one hundred and fifty thousand pounds per square inch and preferably an initial stress of over two hundred thousand pounds per square inch. The concrete should be arranged so as to have an average compressive stress across its section ad jacent the reinforcing steel of at least seventeen hundred pounds per square inch and preferably over four thousand pounds per square inch. After normal plastic flow and shrinkage has occurred in the concrete and normal creep has occurred in the reinforcing rod, there will be at least 120,000 -1b./sq. in. tension stress left in the reinforcing rod andpreferably around 170,000 lb./sq. in. or more. This, of course, will leave proportionately high compressive stresses in the concrete.

To-achieve these desirable results, it has been found advantageous to employ certain types of alloy steelbars andto heat treat them in a particular way to increase their tensile strength and greatly increase their yield point and proportional limit. Also, it has been found desirable to use certain types of concrete mixture and certain procedures for compacting anddensifying the concrete. It has further been found desirable to use certain types of end anchorages for the high strength reinforcing bars to hold the barswithout weakening them and to distribute the stress in the concrete in a way that will prevent injury or crushing of the concrete. Still further, it has been found desirable to prevent bonding of the concrete to the reinforcing rods, prior to tensioning, so that the compression applied by the rod will be uniformly applied.

Considerable variation may be made in these details without departing from the broader aspects of this invention which is the production of a concrete object reinforced by reinforcing bars under a tension of at least 120,000 lbs/sq. in. and placing the concrete, under an average compression, adjacent the bars, of at least 1700 lbs/sq. in., after plastic flow and shrinkage has taken place in the concrete and creep has taken place in the bar.

In accordance with this invention, the concrete objects are comprised of three essential parts, namely, the concrete itself, the reinforcing bars and the end anchorages that transfer the load of the reinforcing bars to the concrete.

In order to obtain concrete that will stand the expected compressive stresses, it is preferred to use a high quality concrete having a 28-day cylinder test of 4000 pounds per square inch or more. This concrete is placedin the forms and vibrated externally or internally or both so as to produce a highly dense, high quality concrete.

The reinforcing rods are preferably of an alloy steel having a composition:

Percent Carbon; 0.38-0.43 Manganese 0.60-0.80 Phosphorus 0.040-maximum Sulphur 0.040-maximum Silicon 0.20-0.35 Nickel 1.65-2.00 Chromium 0.70-0.90 Molybdenum 0.20-0.30

and are heat treated under careful temperature control by a continuous electric induction process. In the heat treating process the steel rods are fed at a predetermined constant speed through a series of electric induction coils and quenching chambers. In the first section the steel is uniformly heated across the, section to a temperature of approximately 1675" F. It is then quickly cooled by quenching to room temperature and again reheated to a temperature of approximately 450 F. and then allowed to cool slowly to room temperature.

The resultant bars are characterized by having a highly uniform microscopic structure across their sections; a proportional limit of approximately 160,000 p. s. i.; a 0.1% yield-point of approximately 170,000 p. s. i.; 0.1% yield point of approximately 214,000 p. s. i.; 0.2% yield point of approximately 232,000 p. s. i.; Johnson elastic limit of approximately 190,000 p. s. i.; and an ultimate yield point of approximately 274,000

In order that the rods may be prepared for their intended use at the factory and then shipped without danger of injury and ready for immediate use by unskilled labor, it has been found highly desirable to coat the rods with a lubricant coating, such for example as asphalt, and then cover the coating and the rod with a protective plasticsheath of some relatively tough plastic material such as polyethylene or polyvinyl resin.

The plastic covering should be sufficiently thick so that any imperfections in the rod will not cause it to bind in the concrete during therelative movement of the bar and concrete during prestressing after the concrete has hardened around the rod and the rod and its coating and plastic covering are all embedded in the concrete. This means that the plastic covering should be somewhat thicker than the greatest expected imperfections in the rod. Generallyplastic coverings of a tough polyethylene plastic at least of an inch thick are satisfactory.

The end anchorages for the reinforcing rods preferably consist of a multi-fianged cup-shaped housing or pressure-distributing me her to distribute the pressure bothlatcrally and axially in the concrete, and in this housing a series or stack or nest of metal plates having aligned holes therethrough, and a plurality of rod-gripping prongs extending from at least some of the plates inwardly toward the center of the aligned hole but angled from the radial direction towards one axial direction so they will grip the rod tighter as it is pulled against them. These prongs are separated so they will act in.- dependently to grip therod and have sufficient resiliency so as to distribute the load between them. Usually the plates carrying the prongs will be separated by spacing washers and it is often desirable touse a pressure cell under the washersto determine the amount of stresson the rod. The bottom of the housing or pressure-distributing plate preferably has a small flange around the hole into which the reinforcing rod extends, so that the plastic cover on the rod may pass over this flange and be secured in place to prevent concrete from entering the hole around the reinforcing rod.

The multiple 'flange end anchorage unit is designed to transfer safely a force exerted by a bar, cable, or strand to concrete within a relatively small: area surrounding 4 the bar or cable at the ends of a structural concrete member.-

Whereas normally anend 'anchor unit consists of only one plate which by virtue of its stiffness distributes the unit described herein engages the concrete by a number of plates or flanges arranged along the axis of the anchor unit in such a way that a greater portion of concrete is utilized for the transfer of forces within reasonably safe stresses.

This accomplished by engaging the concrete in depth rather than in area only. The basic concept of the invention is to create deliberately a separate pressure bulb under each flange or plate rather than depending only on one such pressure bulb created by one single end anchor plate. I

In order to accomplish a reasonable distribution of the total transfer load on a series of load transfer flanges or plates, it is essential to deliberately increase the stiffness of the transfer flanges progressively from the side Where the bar or cable force is applied to the end anchor unit toward the other side of the unit. This can be brought about by a progressive reduction of the diameter of the transfer flanges in a certain ratio, or by a progressive increase of their thickness away from the loaded side, or both.

The underside or inside face of the flanges or plates may preferably be a shape forming an angle with the longitudinal axis of the anchor unit such as a flat cone or a similar shape tending to confine the compressed concrete under each flange and exerting a slight pressure towards the axis of the anchor unit.

The method of transfer of the bar or cable force to the anchor unit can modify the sequence of increase or decrease in stiflness of the transfer flanges, but in principle they should be arranged in such a manner as to individually contribute to the total transfer force as much as their stiffness and the size of the underlying pressure bulb permits.

The distance between individual flanges must be made sufliclent to allow the proper placing of dense concrete.

The new end anchor unit dilfers from earlier attempts of utilizing concrete in depth for transfer of load by not depending on bond between concrete and anchor unit. Its action is that of a multiple anchor plate rather than that of a bonded pipe sleeve. While good bond on the anchor unit surfaces is desirable, its existence is not essential for the performance of this new type end anchor unit.

Details of this construction, and of an alternate construction, will be apparent from the appended drawings and the following detailed description thereof.

In the drawings:

Figure l is a plan view, in section, of a concrete slab prepared in accordance with this invention;

Figure 2 is an enlarged view of a part of Figure 1 showing a section through one of the anchoring members with one end of one of the reinforcing rods held therein;

Figure 3 is a sectional view taken on line 3-3 of Figure 2;

Figure 4 is a perspective view of a modified form of rod anchoring device;

Figure 5 is a detailed sectional view of the device of Figure 2, modified to incorporate the rod anchoring devices of Figure 4;

Figure 6 is a view similar to Figure 2, but showing a ball-type of rod anchor;

Figure 7 is a sectional view of the device shown in Figure 6 taken on line 7-7 thereof; and

Figure 8 is a sectional detail view of the device shown in Figure 6, taken on line 8-8 of Figure 7.

As illustrated in Figure l, a concrete slab may be formed by molding and vibrating in the usual manner, using a high density concrete mixture consisting of Cement 94 lbs. (1 bag). Fine aggregate 140 lbs. maximum size A".

ing 21 of vinyl resin or the like.

Coarse aggregate 230 lbs. maximum size 1";

During the molding of this slab, end anchorages 11, 12 r 13, 14, 15 and 16 may be molded into the slab as shown with reinforcing bars 17, through and also molded into the concrete slab.

These reinforcing bars are preferably coated with asphalt 20 and then covered with a tough plastic coat- This can be seen more clearly in Figure 2. The bars must be prevented from bonding to the concrete, either by a protective covering or by forming holes in the concrete and inserting the bars later.

As can also be seen in Figure 2, the tough vinyl plastic cover has been cut back from the end of the'rod and slipped over a downwardly depending flange 22 on the end anchorage. It may then be bound in this position by a wire 23. This will effectivelyprevent concrete or water from entering the opening in the end anchorage.

When the concrete is cast or molded, the reinforcing rods 17, 18 and 19 are under no tension. After the concrete has achieved its strength, however, the rods are vtensioned, generally by means of an hydraulic jack and in a manner that will be described in connection with the following description of the end anchoring means.

As shown in Figures 2, 3 and 4, the end anchoring means preferably comprises a cup-shaped load distributing member 34). This member has a hole in its inner end to receive a reinforcing rod and it has three outwardly extending flanges 31, 32 and 33 of progressively greater diameter. It is generally a high-grade steel casting and the three outwardly extending flanges bear against the concrete at different axial levels, distributing the load upon the concrete axially as well as radially. By so distributing the load an important advantage is gained in that the concrete is able to carry a far greater load without crushing.

A sheet metal shield 34 attached to the outer flange of the load distributing member serves to position the end anchorage in the form while the concrete is being cast and also serves to prevent the concrete from flowing around the outer end of the anchorage.

Within the cup-shaped load distributing plate 30 there is positioned a stack or nest of pronged plates 35. These are separated by a series of washers 36. The arrangement is. such, as is shown in Figure 4, that each pronged plate carries a series of inwardly extending prongs 37.

The prongs do not extend directly toward the center of the plate, however, but are angled, as shown in Figure 2 so that a rod 17 may be pushedthrough the holes in the plates in one direction but will be retained and held strongly against movement in the other direction. These projecting prongs 37 are preferably wider at the base and at the apex so that they will be strong and yet they are not too heavily constructed to be slightly flexible so that the load will distribute itself throughout the prongs of the entire stack of plates.

As illustrated in Figure 5, it is usually desirable to make the pronged plates of a slightly concavo-convex shape rather than flat. If the pronged plates are assembled along with the washers in nested form as shown in Figure 5 and the opening between the prongs is large enough to allow rod 17 to slip through it rather easily, the plates may then be closed so that the prongs press tightly against the rod 17 by screwing down a pressure cap 38 that is threadedly fitted into the end of the load distributing member 30. Screwing down the cap 38 presses the washers and the rims of the plates 35 tightly together and flattens the rims of the plates 35 thus tilting the prongs 37 toward a radial direction and causing them to grip the rod 17. i

Quite often it is desirable to incorporate a pressure ring 40 at the inner end of the stack or nest of pronged 18 and 19 extending thereplates 35so asto 'be able' to measure the pressure exerted by these plates against the" load--distributing-membef30. This is of cOurSe, a measure of the tension in the reinforcing Tod-17'. A pressure "ring40'can be seen in Fig-j ure- 2"'and"it'isconnected"bya tube 41to a point outside of the strueturl Thereit is normally closed by a cap 42 whichmay'be removed for the'purpose of making pressure readings. These are made in the usual way by a pressure source an'd a pressure gage.

After a 'con'crete member has been cast with the reinforcing-rods fand endanchorages in place as indicated in Figures 1and'2, a hydraulic jack is applied to one or'both' en'ds' oftheexposed rods and these rods are pulled by the hydraulic jack to an extent sutficient to place 'thes rods-under thedesired tension. As can be seen'in' Figure 1, there is" a suffici'ent amount of the rod 17 extending through theends'of the anchoring members 11 andY'12 'to permitthe rods to be grasped by the desiredhydraulidequipment. After. the tension has been placed on the rods, they are generally cut off at a point adjacent the end anchorages and the space that remains is filled with grout as indicated at the ends of the rods 18'and '19 iHiFiglll'C 1..

A modification ofthe end. anchorage shown in Fig ures .2, 3 4 andS may be formed as shown in Figures 6, 7 and-8. In accordance with this modification, the pronged discs 35 arereplaced by a series of balls 50, 51 and '52, and their counterparts at other angular positions about the rod, as shown in Figure 7.

In this construction, the load distributing member 55 is similar to the load distributing member 30 shown in Figure 2 except that the inside of the member is formed with an axial openingjust large enough to accommodate the shaft 56 and a series of axially extending grooves 57, 58, 59 and 60 are cut into the walls of this opening and each is shaped to accommodate a series of balls 50, 51 and 52, or their counterparts. The bottom of the groove 57, and .the other grooves like it, is saw-toothed as is shown in Figure 6 so that balls placed in the grooves will tend to move outwardly against the rod 56 as rod 56 moves downwardly and pulls the balls downwardly withit.

The balls 50, 51 and 52 are urged against the rod 56 by springs 61, 62 and 63, respectively, each of which has its upperend against a saw-tooth in the bottom of the groove57 "and its bottom end against its particular ball. Theballs are prevented from leaving the groove 57 by in-turned edges 64 and 65 which are pressed into the position shown inFigures 7 and 8 after the balls and springs are in place. These in-turned edges 64 and 65 preventthe escape of the balls but they are not pressed inwardly'far enough to prevent the edges of the balls from extending into the axial bore in the load bearing plate and clamping the rod 56 therein.

Considerable attention has been given to the method of anchoring the reinforcing rods at their ends and this is not-without reason. In order to make rod reinforcingeffeetive and. efiicient, it is necessary to place the rods, initially, under a very nearly maximum tension. Any attempt. to. do this by merely threading the end'of the rod and placinga nut thereon results in a weakened area at the end j of the rod'iand also results in "the p1 aeing 'of" the tension-011 the first tewithreads." As a result,

maximum" tension in the remainder'of the'rod can fnot be develo'pe'd andthe' reinforcing steel is far less efficient than'if maximum tension were placed on it. This is not a simple proportional loss in effectiveness of the reinforcing steel, ascan' be illustrated by the following example.

Under'a'n'ormal set-of circumstances, the reinforcing steel will lose approximately 30,000 lbs./ sq. in. of tension during the stabilizingprocess,by reason of the shrinkage and plastic how of the concrete and the creep of the steel. If the steel were'placed under only 30,000 lbs. of tension tobegin with, it would have practically zero tension in the final construction. If it were placed under 60,000 lbs. of tension, it would have approximately 30,000 lbs. in the final construction, and then if the tension were again increased by.a factor of only 50% to make it 90,000 lbs.-, the final tension would be doubled to make it 60,000 lbs/sq. in.

This application is a division of application Serial Number 239,423, filed by Curzon Dobell on July 31, 1951, and now abandoned.

What is claimed is:

l. A reinforced structure comprising a body of concrete and at least one stressed steel rod disposed therein, said rod being free to move with respect to the concrete body, a hollow housing embedded Within the body adjacent a surface thereof and an end portion of said rod disposed within said housing, a stack of metallic plates positioned within said housing, said stack having an aligned hole therethrough and said rod portion being disposed within said hole, at least some of said plates having a plurality of separate rod-gripping prongs integral thercwith and extending inwardly toward said surface and resiliently bearing against said stressed rod, said prongs being sufiieiently resilient to distribute the tensional load on the rod between them.

2. A reinforced structure asreEited in claim 1 wherein said housing includes a plurality of spaced, radially extending flanges on its exterior surface, said flanges being angled toward the center of said body and distributing the load on said housing into the surrounding concrete.

References Cited in the file of this patent UNITED STATES PATENTS 1,352,201 Kennedy Sept. 7, 1920 1,455,641 Mahon May 15, 1923 1,629,880 Mirzan May 24, 1927 1,688,087 Mirzan Oct. 16, 1928 1,818,254 Hewett Aug. 11, 1931 FOREIGN PATENTS 338,864 Great Britain Nov. 18, 1930 900,581 France Oct. 9, 1944 OTHER REFERENCES Stressteel Tensioning Units-Edition, March 1953, No. 6-90 by Truscon Steel Division-Republic Steel Corp. Pages 1, 2 and 14. 

